In certain known types of electronic systems, particularly those designed for electronic article surveillance (EAS), it is known to provide a composite antenna comprising two or more antennas coupled to each other and to which signals from a transmitter are supplied in order to produce a surveillance zone comprising an induction field adjacent the composite antenna which is sufficiently strong to detect the presence near the antenna of predetermined types of objects. Generally, EAS systems include both a transmit antenna and a receive antenna which collectively establish a surveillance zone, and tags which are attached to articles being protected. The transmit antenna generates an electromagnetic field within a range of a first predetermined frequency. The tags each include a resonant circuit having a predetermined resonant frequency generally equal to the first frequency. When one of the tags is present in the surveillance zone, the field generated by the transmit antenna induces a voltage in the resonant circuit in the tag, which causes the resonant circuit to generate an electromagnetic field, causing a disturbance in the field within the surveillance zone. The receive antenna detects the electromagnetic field disturbance and generates a signal indicating the presence of the tag (and thus, the protected article attached to the tag) in the surveillance zone.
In order to avoid the production of relatively strong electromagnetic fields which might interfere with the operation of other electronic apparatus, it is desirable to design such EAS systems so that the net effect of the radiated fields at positions remote from the antennas (generally is thirty meters) substantially zero, or at least insufficient to cause any serious problem.
In order to provide the desired far field cancellation, it is known to construct the composite antenna of a plurality of loops, the planes of which are substantially parallel and adjacent but displaced from each other, and in which the transmitter current flow is adjusted in phase and amplitude in different loops, so that the fields produced by the loops essentially add up to zero. Using such a composite antenna, it has been found possible to provide canceling in the far field by suitable choice of the cross-sectional areas and numbers of turns in the several loop antennas.
Although such composite antennas provide far field cancellation, the orientation of the magnetic fields due to the respective loop antennas are essentially constant at any point in space proximate to the antennas. For example, in the case of two loops of equal area offset from each other in the plane of the loops such that a figure-8 current path is described (i.e. one loop having a clockwise current direction and the other having a counter-clockwise current direction) and wherein the number of ampere turns in the loops are equal, a zone will exist in a second plane perpendicular to the plane of the loops and passing through a mid-point between the two loops wherein the field orientation is perpendicular to the second plane. However, essentially no field will exist in any direction within the second plane. In an EAS system, this substantially reduces the coupling probabilities and substantially inhibits the coverage of the surveillance zone.
The present invention provides an antenna including at least one active loop for generating and responding to fields located proximate to, and preferably within a larger, passive loop, which substantially cancels the far fields generated by the active loop or those far fields to which the active loop is responsive. By providing a separate loop for reducing far field coupling, the active loop may be driven by a transmitting circuit with a relatively high current, while still meeting regulatory requirements for far field radiation. This allows for an antenna with relatively larger fields in more than one orientation within close proximity to the antenna. It also allows for a separate receive antenna which is highly sensitive to externally emitted signals to be placed proximate the active and passive loops.
An antenna of the present invention may be used to advantage in systems where simultaneous transmission and reception occurs, or where it is desired to phase different elements of the antenna in different phases and/or frequencies. Providing different phases allows the antenna to strongly couple in more than one direction at a point.
The antenna may be used in an EAS system and provides larger fields in as many orientations as possible within as close proximity of the antenna as possible, thereby causing a tag to respond, and simultaneously providing an antenna pick-up pattern which is also uniformly sensitive to emitted radio frequency (RF) signals such that the tag responses can be sensed.